Field of the Invention
Exemplary embodiments of the present invention relate to a gallium nitride-based semiconductor device, and more particularly, to a method of fabricating a nonpolar gallium nitride-based semiconductor layer, a nonpolar semiconductor device, and a method of fabricating the same.
Discussion of the Background
Gallium nitride-based compounds are recognized as important materials for high-power high-performance optical devices or electronic devices. In particular, since group-III nitrides, such as GaN, have excellent thermal stability and a direct transition energy band structure, group-III nitrides have recently attracted much attention as materials for light emitting devices of a visible ray region and an ultraviolet ray region. For example, blue and green light emitting devices using InGaN have been utilized in a variety of applications, for example, large-sized natural-color flat panel display devices, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, and optical communications.
However, since if is difficult to fabricate homogenous substrates capable of growing the group-III nitride semiconductor layers thereon, group-III nitride semiconductor layers have been grown on heterogeneous substrates having a similar crystal structure through metal organic chemical vapor deposition (MOCVD). As the heterogeneous substrates, sapphire substrates with a hexagonal structure have been mainly used. In particular, since GaN epitaxial layers tend to be grown with a c-plane orientation, sapphire substrates with a c-plane growth surface have been mainly used.
However, an epitaxial layer grown on a heterogeneous substrate has a relatively high dislocation density due to lattice mismatch and thermal expansion coefficient difference with respect to a growth substrate. It is known that an epitaxial layer grown on a sapphire substrate generally has a dislocation density of IE8/cm2 or more. Such an epitaxial layer having a high dislocation density has a limit to improving the luminous efficiency of light emitting diodes.
Furthermore, a c-plane gallium nitride-based semiconductor layer grown on a c-plane growth surface generates an internal electric field due to spontaneous polarization and piezoelectric polarization, which reduces a radiative recombination rate. In order to prevent such polarization phenomenon, research into nonpolar or semipolar gallium nitride-based semiconductor layers is in progress. As one of such research, attempts have been made to form a gallium nitride layer using a nonpolar or semipolar gallium nitride substrate as a growth substrate. However, in a case where a gallium nitride layer is grown on a nonpolar gallium nitride substrate using a growth method on a sapphire substrate, the gallium nitride layer has a very rough surface morphology. In a case where a semiconductor device, such as a light emitting diode, is fabricated using such a gallium nitride layer, a leakage current is large and a nonradiative recombination rate is increased, making it difficult to obtain excellent luminous efficiency.